Natural convection induced by unintended horizontal temperature distribution in a narrow-closed container heated from above

•The mean velocity of the liquid held in a narrow-closed container heated from above decreases owing to the decrease in the unintended horizontal temperature distribution as the inner cavity of the container.•Reynolds number of the liquids are expressed as an exponential function of the Grashof number irrespective of the external thermal and dimensional conditions.•The diffusion-dominant condition in the liquids was suggested as the correlation of Reynolds number, Schmidt number, and the geometry of the container.